Chloride Induced Stress Corrosion Cracking (SCC) of Stainless Steel Water Still

A water still failed due to water leaks in the shell, which leaks were below the water line in an area heated by an internal steam coil. A weld repair was attempted. After welding, several other cracks in the same general area opened up.

The still was disassembled and the outer insulation was stripped off for examination. Several rust stains were observed on the inside of the shell below the tube sheet in the area of the weld repair.

A dye penetrant test performed on the outside revealed several other cracks, where all of these cracks were below the tube sheet. Some of these cracks penetrated through to the inside as evidenced by red penetrant seeping through to the inside of the still. In no instance was a crack found on the inside that was not a through wall penetration. Several cracks were found on the outside with no indication of through wall penetration (Left picture). The cracks had initiated from the outside to the inside.

Two types of insulation were employed on the still. A blanket type covered the majority of the still with an additional insulation tape below the tube sheet in the area where the cracks were found. Samples of each insulation were taken to determine chloride content by extraction. Chloride levels were extremely high. The blanket insulation contained 900 ppm Cl^- and the tape contained 15,000 ppm Cl^-.

The residual stresses (from the welded brackets, supports and nozzles) and the hot humid environment combined with the chloride levels of the insulation provided the necessary conditions for chloride induced stress corrosion cracking to occur.